There is a need for low cost compact instrumentation which can accept frequency input signals having any wave form and frequency and convert it to a DC current whose magnitude is precisely related to the input frequency. This suggests a two-wire system wherein the power input leads also carry the output signal. Such an arrangement is compact and easy to install with little chance for error in installation because the number of leads to be connected is minimized.
Considerations of cost make it desirable for the frequency to DC instrumentation to be constructed without regard to the input frequency. However, design considerations require each circuit to be constructed to receive a particular input frequency range. These contradictory demands are reconciled in this case through the use of a clock controlled binary counter. This is because such counters have a plurality of output stages, each associated with a particular frequency.
In a binary counter, a square wave pulse appears at the output terminal of each stage when the count of pulses from the clock or oscillator to the binary counter has achieved a number corresponding to the stage.
The circuit is designed to these various output stages lead into separate standardized open terminals. These terminals are designed to be completed by the connection of a single diode across them. By connecting the diode across a particular open terminal, the circuit will be customized to receive a particular input frequency range.
Since the frequency input can have a wide variety of wave forms, considerations of accuracy, and sensitivity to temperature variation, and compactness, suggest the use of a crystal oscillator such as described in the 1976 printing of the National Linear Data Book, published by National Semiconductor Corporation, pages 5-33, to be used with the binary counter. Similarly, accuracy requires a highly stable voltage supply, which is likewise independent of frequency and temperature. That is satisfied in this case through the use of a power supply employing, inter alia, a Zener diode. The Zener controlled power supply is used to achieve a highly stable voltage supply which is independent of temperature. Thus, the combination of a crystal oscillator, a highly stable Zener controlled power supply and a binary counter makes possible a very accurate transformation of a frequency input to a DC output.
What is needed, therefore, and comprises an important object of this invention is to provide a two-wire frequency to DC converter which has a standardized output value and which can convert an input frequency having a large variety of wave forms to a highly accurate DC output.
A further object of this invention is to provide a single two-wire frequency to DC converter which can be massproduced but which can be easily customized for a particular input frequency range.